Deficiency of the nutritionally important amino acid, taurine results in abnormalities in development of the central nervous system, retinal degeneration decreased myocardial function, and problems with feline pregnancy and outcome. Our preliminary data indicate that these changes are accompanied by abnormalities in the immune system. Specifically, the results show that a lack of taurine in the diet results in a significant decrease in total white cells, a shift in the percentage of polymorphonuclear and mononuclear cells, and a change in the sedimentation characteristics of white cells. Functional studies of polymorphonuclear isolated from cats fed taurine-free diets show a significant decrease in peroxide production as well as a decrease in phagocytosis and intracellular killing of Staphylococcus epidermis compared to cats fed the same diet containing taurine. In addition, serum gamma globulin in cats fed taurine-free diets was significantly increased compared to taurine-supplemented cAts indicating that other immune cells may be affected by taurine- deficiency. Pathology of the spleen revealed a depletion of follicular center reticular cells, mature and immature lymphocytes (B cell areas) and a depletion of periarterial lymphocytic sheaths (T cell areas), as well as ongoing mild extravascular hemolysis. We propose to explore and define the role of taurine in the immune system by using the cat model of taurine deficiency. Cats have been shown to be exquisitely dependent on a dietary source of taurine to maintain their body taurine pool. The number and functions of peripheral blood leukocytes as well as assessment of the distribution of lymphocyte sub-populations will be studied in cAts fed taurine- deficient and taurine-supplemented diets. Sequential studies on peripheral blood leukocytes from cats fed a taurine-free diet after a diet containing taurine will establish the onset of changes in the immune response. In addition, refeeding of taurine will establish whether these changes can be reversed after onset. Cats will be killed at selected time for analysis of leukocyte numbers and function using both blood and lymphoid organs. In addition, taurine concentration in tissues will be measured and samples taken for routine pathology. These studies in the cat model of taurine deficiency will help to define the role of taurine in the immune system.